The modem is designed to always stay plugged in and powered on. If you power it off there is a chance you will lose the settings you have configured.

To disable the wireless follow these steps:

1. From a computer connected to the unit by ethernet, open your browser and type 192.168.0.1 in the Address Bar, hit enter.
2. Click Manual setup if the setup menu appears, if not, click the Wireless Setup icon.
Make sure Wireless is on.
3.Set the wireless radio state to disable.
4.Click apply at the bottom of the page.

You are done! The wireless will now be disabled until you go back and enable the wireless radio using these steps but selecting enable instead. Can we ask why you are disabling the wireless?